The invention relates to a filtering device for highly viscous media.
Such filter devices are known from DE 102 54 022 A1 or DE 103 17 170 A1, for example. In particular, they have either two screen plugs each with a screen cavity or one screen plug having two screen cavities. As a result, it is possible to keep one of the screen cavities in production operation, while the other one is being cleaned. Such a filtering device therefore allows a continuous operation of an installation, in which a highly viscous medium, particularly a plastic melt, has to be cleaned.
Although the known filtering devices have proven useful, a partial material degradation can occur, particularly when filtering plastic melts, leading to the partial carbonization of the melt on the clean side of the screen, which causes defects precisely in the case of transparent plastics. These problems occur particularly if larger filter surface areas are required due to small installation throughputs, because the aim is to achieve a particularly fine filtration.
These degradation processes are explained by the fact that the medium to be filtered has to be kept hot in the filtering device, in order to keep the viscosity of the medium as low as possible, and thus be able to push the medium through the filter inserts at a relatively low pressure.
In the normal uninterrupted throughput at a defined flow rate, the influence of the temperature does not yet have a decomposing effect. However, the above-described defects can occur if, given certain rheological properties of the medium, flow zones form within the flow channels, in which zones the melt remains for a longer time and is exposed to the influence of the temperature.
The two known variants of continuous screen changers always require several and also larger flow canals which together have a relatively large volume, which in turn entails a lower flow rates or higher residence times of the medium.
Besides the considerations relating to flow technology, the manufacturing costs play a role. As a rule, continuously operating screen changers have two screen plugs each with one screen cavity. For this purpose, larger housings have to be provided, and two fittings between the housing bore and the screen plug have to be established. Moreover, two hydraulic units are required to shift the screen plug.
The problem of the present invention therefore is to reduce such residence zones, also referred to as dead water zones, in order to achieve the most consistent possible rinsing through of the entire volume area within the screen cavities, and to achieve a consistent residence time for all the current filaments of a flow through a screen plug. An additional problem is to allow a more cost effective construction of a continuously operated filtering device.